1. Field of the Invention
This invention relates to data recording. More specifically, the present invention is directed to a self-clocking digital recording apparatus.
2. Description of the Prior Art
The conventional method of extracting a clock signal from a playback of a recorded self-clocking digital code, such as bi-phase and phase modulated, is to use a locally generated fixed time period pulse in a clock signal extracting circuit to overlap the unwanted reproduced data pulses. This technique blanks out the unwanted data pulses in the clock extraction circuit while supplying a fixed clock pulse from the reproduced coded signals. In conventional self-clocking codes, there is always a transition of the recorded level at a time when a clock pulse has been completed regardless of the information content of the data being concurrently recorded. Whether a transition also occurs between these regular "clocking transitions" depends upon the data, or information, recorded along with the clock signals. A negative going transition at the clock transition time generally denotes a binary 0 and a positive going transition generally denotes a binary 1. Thus, the recorded clock signal is present during play back in the clock extraction circuit along with the pulses which were recorded as a result of the information content. The blanking pulse is ordinarily located in an unblank position midway between the unwanted data pulse and the desired clock signal transition. The time between the termination, or return, of the blanking pulse and the adjacent reproduced data pulses is a safety zone. This safety zone would normally be sufficient to insure that the unwanted data pulses are disregarded by the clock extraction circuit were it not for so-called tape jitter. At the packing density in bits per inch of tape per track is increased, the tape jitter begins to consume a substantial portion of the safety zone by producing displaced data pulses until eventually errors in the extracted clock signal occur. If this happens before the maximum packing density defined by the magnetic limitations of head gap, tape properties and tape speed has been reached, the maximum storage efficiency of the type recording is unavailable. Accordingly, an alternative approach to clock recovery which minimizes the effect of tape jitter is desirable to eliminate errors in the extracted clock signal.